JP2010256165A - Photoionization detector and method of detecting photoionization - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photoionization detector and a detecting method, reducing a decrease in measurement sensitivity and performing precise, maintenance-free measurement for a long time. <P>SOLUTION: The photoionization detector has: a detection electrode 2 of VOC in a measurement fluid; an AC application circuit 3 for applying an AC voltage or an AC current to the detection electrode 2; a UV lamp 4 for irradiating the measurement fluid with ultraviolet rays; an excitation circuit 5 of the UV lamp 4; and a measurement circuit 7 for measuring current or voltage flowing to the detection electrode 2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a photoionization detector and a photoionization detection method, and more particularly to a photoionization detector and a photoionization detection method capable of reducing a decrease in measurement sensitivity and capable of high-accuracy measurement.

Volatile organic compounds such as toluene and xylene (hereinafter referred to as “Volatile Organ-ic Compounds”: VOC) cause photochemical smog and suspended particulate matter, and adversely affect the living environment in the vicinity of emission sources such as factories. . Therefore, VOC reduction is an important issue especially in the vicinity of the discharge facility and in the city center.
In order to reduce VOCs, regulations based on the Air Pollution Control Act and the Environment Assurance Ordinance have been established. However, in order to achieve both environmental conservation and industrial promotion, further reduction technology development is required. In recent years, as interest in the environment has increased, the need to measure VOCs has increased, and a technology that can measure VOCs more accurately and accurately is desired.

Examples of the VOC measurement device include a flame ionization detector (official method), a catalytic oxidation / non-dispersion infrared analyzer (official method), a semiconductor sensor, a catalytic combustion type sensor, and a gas chromatography. Among them, the flame ionization detector is used as an official method, and it is simple and can measure VOC accurately. However, since installation of a hydrogen cylinder or the like is necessary, there is a problem that the apparatus is complicated and the apparatus itself is expensive. Further, a catalytic oxidation / non-dispersion type infrared analyzer is also used as an official method, but, like a flame ionization detector, there is a problem that a large apparatus is required and the apparatus itself is expensive.
In addition, the sensitivity loss due to contamination, the output value is unstable due to drift, the problem that sensitivity difference occurs due to concentration, the loss of sensitivity, drift, and low concentration (100 ppm or less) are also the same for the catalytic combustion type sensor. In addition, the gas chromatography has a problem that it is expensive because the gas chromatography is a large apparatus and cannot be continuously measured.

  On the other hand, in addition to these devices, a photoionization detector (measurement method using a photoionization sensor) as a VOC measurement device illustrated in FIG. 9 is known. The measurement principle of the photoionization detector (PID) is that a measuring fluid is guided into a device in which a pair of application electrodes are arranged, and ultraviolet rays (UV) of a short wavelength are irradiated to ionize VOC, and the ions are applied to the application electrode. The detection current proportional to the VOC concentration can be obtained by capturing with. That is, since ions charged by UV are guided to the electrode and the current value between the pair of applied electrodes changes, the VOC concentration can be measured by converting the current value to the VOC concentration.

  The photoionization detector includes a detection chamber 1 into which the VOC measurement fluid FL is introduced and discharged, a metal electrode 2 provided in the detection chamber 1, and a DC application circuit 13 that applies a DC voltage to the metal electrode 2. , A UV lamp 4 that irradiates the measurement fluid in the detection chamber 1 with ultraviolet light of a short wavelength, an excitation circuit 5 that excites the UV lamp 4, and a measurement circuit 7 that measures the current flowing through the metal electrode 2 (for example, an ammeter A) And an arithmetic unit 8 (control device) for performing a calculation for converting the current value into the VOC concentration.

In this conventional photoionization detector, the VOC measurement fluid FL is introduced into the detection chamber 1 and discharged from the detection chamber 1 after being irradiated with ultraviolet rays by the UV lamp 4. The detection chamber 1 is provided with a metal electrode (direct current application electrode), and the introduced measurement fluid FL is ionized by ultraviolet rays irradiated by a UV lamp 4 provided on the side wall of the detection chamber 1, and this ion Alternatively, an electric current is generated in the application electrode 2 by attracting the electrons to the metal electrode 2 and capturing them.
Then, this current is measured by the measurement circuit 7. Further, the current value is output as a VOC concentration by multiplying the coefficient of each substance (measured VOC substance) by the calculator 8. Although such a photoionization detector is not an official method, it is more suitable for measuring a specific VOC because it has selectivity for each VOC component compared to the other detectors and detection methods described above. In addition, it is an effective method for most VOC measurements, and since the apparatus is not large in size, it can easily measure VOCs and is a very useful method for measuring VOCs.

However, in the conventional photoionization detector, (1) contamination such as an insulator accumulates on the surface of the metal electrode due to long-term use, and the arrival of the ionized VOC to the electrode is hindered, resulting in a decrease in sensitivity. In addition, (2) when the metal electrode is covered with a contaminant, there is a problem that the sensitivity is lost and the sensor does not function as a sensor.
Therefore, if contaminants accumulate on the surface of the metal electrode, it becomes impossible to obtain reproducibility, so that periodic electrode maintenance is required.
Therefore, according to Patent Document 1 below, the volatile gas concentration (corresponding to the VOC gas concentration) is continuously measured, and oxygen in the ionization detection chamber is converted into ozone by a UV lamp, whereby ionization detection is performed by the ozone. A configuration for self-cleaning of PID to remove indoor dirt is disclosed.

Patent Document 2 below discloses a photoionization detector having a basic configuration substantially similar to that of the photoionization detector illustrated in FIG. Specifically, the photoionization detector of Patent Document 2 is provided with a transmission window and an application electrode, a detection chamber into which a measurement fluid is introduced and discharged, a lamp that irradiates ultraviolet light of a short wavelength, and an excitation that excites the lamp. The circuit includes an application circuit that applies a voltage to the application electrode.
And in the photoionization detector of patent document 2, in order to prevent the fall of a sensitivity in addition to such composition, while operating a lamp intermittently and making a measurement fluid retain in a detection chamber in synchronism with it, A configuration is disclosed in which generation of causative substances such as radicals is suppressed and sensitivity is prevented from being lowered as compared with a case where a measurement fluid is continuously introduced.

Non-Patent Document 1 below describes the basic configuration of a photoionization detector. Specifically, UV generation necessary for ionizing a polyatomic compound and introduction of a measurement fluid into a detection chamber are described. And the configuration of the electrodes. In addition, discharge using rare gas, nitrogen, or hydrogen can generate photons with sufficient energy to ionize polyatomic compounds (VOC, etc.), and chamber type anode (C) for measuring electrons. It is disclosed that the introduction pipe D (cathode) for the measurement fluid surrounded by the liquid crystal also functions as a collector electrode for liberated ions.
According to Non-Patent Document 1, since the metal electrode of the photoionization detector is composed of a cathode D (cathode) and an anode C (anode) and the direction of current flow is determined, FIG. It can be understood that the metal electrode 2 in the conventional photoionization detector exemplified in No. 1 and Patent Document 2 below is a DC application electrode.

JP 2003-66008 A JP 2003-98153 A J. E. Lovelock, Chemistry, Gas / Vapor Photoionization Detector for Gases and Vapors, Nature, No. 4748, October 29, 1960, page 401

The configuration described in Patent Document 1 can measure the concentration of volatile gas molecules in real time and self-clean the PID, but it is complicated to control the gas detection unit so as to convert oxygen contained in the surrounding gas into ozone. It is a configuration. Further, since contamination such as an insulator accumulates again on the surface of the metal electrode even after self-cleaning, periodic cleaning is still necessary.
Even in the configuration described in Patent Document 2, when it is used for a long time, contamination still accumulates on the surface of the metal electrode, so that maintenance is required.

As described above, when the contamination accumulates on the surface of the metal electrode, the measurement sensitivity decreases. Therefore, depending on the type and concentration of the VOC to be measured, it is not possible to detect a signal smaller than noise due to ionization.
An object of the present invention is to provide a photoionization detector and a photoionization detection method capable of reducing a decrease in measurement sensitivity and capable of long-term maintenance-free and high-precision measurement.

The invention according to claim 1 is a detection electrode for detecting a volatile organic compound in a measurement fluid, an application means for applying an alternating voltage or an alternating current to the detection electrode, and ionizing the volatile organic compound in the measurement fluid. Photoionization detection of a volatile organic compound having a UV lamp for irradiating the measurement fluid with ultraviolet light, an excitation circuit for exciting the UV lamp, and a measurement means for measuring a current or voltage flowing through the detection electrode It is a vessel.
A second aspect of the present invention is the photoionization detector according to the first aspect, wherein the measuring means is a phase detector.
A third aspect of the present invention is the photoionization detector according to the first or second aspect, wherein the detection electrode is covered with an insulating coating.

According to a fourth aspect of the present invention, an AC voltage or an AC current is applied to a detection electrode for detecting a volatile organic compound in a measurement fluid, and the measurement fluid is irradiated with ultraviolet rays to volatile organic compounds in the measurement fluid. Is a photoionization detection method for a volatile organic compound, in which a current or a voltage flowing through the detection electrode is measured.
A fifth aspect of the present invention is the photoionization detection method according to the fourth aspect, wherein the current or voltage flowing through the detection electrode is measured by phase detection.
A sixth aspect of the present invention is the photoionization detection method according to the fourth or fifth aspect, wherein the detection electrode is an insulating coated electrode.

(Function)
A conventional means for applying a voltage to the metal electrode (detection electrode) of the PID is a DC application circuit as described in Non-Patent Document 1. In addition, although it is not clearly described in the said patent document 1 and the patent document 2, it is common to use a DC application circuit.
And according to this invention, the means which applies an alternating voltage or an alternating current using an alternating current application circuit is provided as a means to apply a voltage to a detection electrode, It is characterized by the above-mentioned.

  When a DC application circuit is used as in a conventional PID, since the current flowing direction is constant, if contamination accumulates in one of the two DC application electrodes, ions of the DC application electrode or The area where electrons can be captured becomes smaller. Therefore, the current flow is hindered, and an accurate current value cannot be measured, resulting in a decrease in measurement sensitivity. That is, when a conventional DC application circuit is used, contamination has hindered contact between ions and electrodes.

  On the other hand, the generation of current in the AC application electrode occurs not only by the ion current but also by the detection electrode itself acting as a capacitor. Since the dielectric loss of the detection electrode is changed by the ionized VOC, the amount of charge stored in the capacitor is changed, and the change in the current value can be measured. Therefore, even if contamination accumulates on the detection electrode, it is possible to prevent loss of measurement sensitivity by applying alternating current to the electrode.

In general, it is recognized that noise is more easily picked up in the case of alternating current application than in the case of direct current application. Therefore, it is difficult to think of increasing the measurement sensitivity by applying an alternating current. However, the present inventors have intensively researched and found that in the situation where contamination accumulates on the electrode in this way, the sensitivity is lower in the case of DC application than in the case of AC application. The invention has been completed.
In addition, in the case of AC application, even if contamination accumulates in one of the two AC application electrodes, VOC ionized by the other electrode can be captured, thereby preventing a decrease in measurement sensitivity. .

The method of applying and detecting to the electrode is (a) applying a constant voltage to the electrode and performing detection by changing the output (current flowing through the electrode circuit) and (b) applying a constant current to the electrode, Detection may be performed by a change in output (voltage applied to the electrode circuit).
Therefore, according to the first and fourth aspects of the invention, by applying an AC voltage or an AC current to the detection electrode using an AC application circuit, the detection electrode also functions as a capacitor, so that the current value can be measured. is there. Therefore, it is possible to prevent the detection electrode from losing measurement sensitivity due to contamination.

  In addition, depending on the type and concentration of the VOC to be measured, a minute signal change due to ionization may be buried in noise, but by measuring the current or voltage flowing through the detection electrode by phase detection, that is, an AC application circuit If the phase of the applied voltage is synchronized with the phase of the detection signal, noise can be removed. For example, a current signal flowing through the detection electrode may be measured using a phase detector. Noise is removed from the detection signal by synchronizing the detection signal flowing through the detection electrode with the signal of the stable AC application circuit. As a result, the VOC concentration can be measured with high accuracy. That is, according to the second and fifth aspects of the invention, in addition to the effects of the first and fourth aspects of the invention, noise can be removed by measuring the current or voltage flowing through the detection electrode by phase detection. Therefore, it is possible to measure the VOC concentration with high accuracy.

Alternatively, an electrode in which an insulating film is coated on the detection electrode may be used. As a result, a long-term maintenance-free electrode can be obtained.
Since the AC application electrode can detect a change in the capacitance (electric capacity) of the electrode due to the ionized gas, even if an electrode covered with an insulating film is used, the current value can be measured by the change in the capacitance. . Since the insulating film portion coated with the insulating film prevents the electrode from being contaminated by generation of an active substance by UV irradiation or mixing of a corrosive substance, long-term maintenance-free operation is possible. Moreover, even if a metal electrode is used, the metal part of the electrode does not deteriorate.
Therefore, according to the invention described in claims 3 and 6, in addition to the operation of the invention described in claim 1 or 2 and claim 3 or 4, the insulating film portion coated with the insulating film is formed of the active substance by UV irradiation. Since no contamination due to generation or mixing of corrosive substances occurs, contamination does not accumulate in the conductor portion of the electrode, and long-term maintenance-free operation is possible.

The photoionization detector of the present invention can measure the VOC concentration even in the presence of contaminants by applying an alternating voltage or alternating current to the metal electrode.
Specifically, according to the first and fourth aspects of the invention, it is possible to prevent loss of measurement sensitivity due to contamination by applying an AC voltage or an AC current to the detection electrode using an AC application circuit. Accordingly, it is possible to reduce a decrease in measurement sensitivity of the VOC concentration.

According to the second and fifth aspects of the invention, in addition to the effects of the first and fourth aspects of the invention, noise can be eliminated and high accuracy can be obtained by measuring the current or voltage flowing through the detection electrode by phase detection. This makes it possible to measure the VOC concentration.
According to the invention described in claims 3 and 6, in addition to the effect of the invention described in claim 1 or 2 and 3 or 4, the accumulation of contamination can be achieved by using an electrode coated with an insulating film as the detection electrode. It can be prevented and long-term maintenance-free is possible.

It is a block diagram of the photoionization detector by one Embodiment of this invention. FIG. 1A is a diagram in the case where the resistor R is provided in the AC application circuit, and FIG. 1B is a diagram in the case where the ammeter A is provided in the AC application circuit. It is a block diagram of the photoionization detector by other embodiment of this invention. It is a block diagram of the photoionization detector by other embodiment of this invention. It is the figure which showed the measurement result which compared the sensitivity difference of the DC electrode and AC electrode by contamination using the photoionization detector (FIG. 1) by one Embodiment of this invention, and the conventional photoionization detector. It is the figure which showed the result of having measured the electric current value in order to confirm the presence or absence of noise using the photoionization detector (FIG. 2) by one Embodiment of this invention. It is the figure which showed the relationship between the VOC density | concentration and the detected electric current value difference at the time of using the photoionization detector (FIG. 2) by one Embodiment of this invention, and the relationship between a VOC density | concentration and a phase difference. It is the figure which showed the measurement result which compared the sensitivity difference of the direct current electrode and alternating current electrode by contamination using the photoionization detector (FIG. 2) by one Embodiment of this invention, and the conventional photoionization detector. The figure which showed the result of having measured the electric current value in order to confirm the presence or absence of noise similarly to Example 2 (FIG. 5) using the photoionization detector (FIG. 3) by one Embodiment of this invention. is there. It is a block diagram of the photoionization detector of a prior art example.

A photoionization detector according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration diagram of a photoionization detector according to an embodiment of the present invention.
The photoionization detector of FIG. 1A applies an AC voltage to the detection chamber 1 into which the VOC measurement fluid FL is introduced and discharged, the metal electrode 2 provided in the detection chamber 1, and the metal electrode 2. An AC application circuit 3 provided with a resistance R, a UV lamp 4 for irradiating a measurement fluid in the detection chamber 1 with ultraviolet light having a short wavelength, a UV lamp excitation circuit 5 for exciting the UV lamp 4, and the metal electrode 2 are generated. A measurement circuit 7 that measures current and an arithmetic unit 8 that performs calculation for converting the current value into the VOC concentration are included.

  In the present invention, an ammeter A may be connected. In this case, the ammeter A serves as the measurement circuit 7 as shown in FIG. 1A and 1B differ from the conventional photoionization detector of FIG. 9 in that the DC application circuit 13 in the photoionization detector of FIG. .

Here, the present invention will be further described. The VOC measurement fluid FL is introduced into the detection chamber 1 and discharged from the detection chamber 1 after being irradiated with ultraviolet rays by the UV lamp 4. The detection chamber 1 is provided with a metal electrode (AC application electrode), and the introduced measurement fluid FL is ionized by the ultraviolet rays irradiated by the UV lamp 4 provided on the side wall of the detection chamber 1, and this ion Alternatively, an electric current is generated in the application electrode 2 by attracting the electrons to the metal electrode 2 and capturing them.
When an AC voltage is applied to the metal electrode 2 by the AC application circuit 3, the metal electrode 2 functions as a capacitor, so that a current depending on the dielectric constant of air flows through the AC application circuit 3. The current is measured by measuring the voltage across the resistor R. As shown in FIG. 1B, when the current is directly measured by the ammeter A, the resistance voltage is measured inside the ammeter.

The VOC in the measurement fluid is ionized by ultraviolet (UV) irradiation. Then, the current value changes as the electric capacity of the metal electrode 2 changes due to the ionization of the VOC, and when an ion current is generated by drawing ions or electrons to the metal electrode 2.
Then, the current signal flowing through the metal electrode 2 is taken out, the current value before ion generation is subtracted from the current value at the time of ion generation, and the current value due to ionization is measured by the calculator 8. Note that, when a constant current is applied to the electrode 2, the voltage value also changes due to the generation of an ionic current. Therefore, the voltage value between the electrodes 2 due to ionization may be measured by the measurement circuit 7.

Further, the current value is output as a VOC concentration by multiplying the coefficient for each substance by the calculator 8.
Since the electric capacity of the metal electrode 2 is changed by the ionized VOC, the current value can be measured by the change. Therefore, even if contamination accumulates in the metal electrode 2, it is possible to detect ions or electrons, and it is possible to prevent the AC electrode from losing measurement sensitivity due to contamination.

Moreover, even if contamination accumulates in one of the two AC-applied metal electrodes 2, VOC ionized by the other metal electrode 2 can be captured, thereby preventing a decrease in measurement sensitivity. .
Therefore, as shown in this embodiment, by applying an AC voltage or an AC current to the metal electrode 2 using the AC application circuit 3 to the metal electrode 2, the metal electrode 2 also functions as a capacitor. Voltage value) can be measured. Therefore, it is possible to prevent the metal electrode 2 from losing measurement sensitivity due to contamination.

Examples of measurable VOCs include various substances published by the Ministry of the Environment, such as alkanes, alkenes, alcohols, ethers, aldehydes, carboxylic acids, and aromatic hydrocarbons. Examples of particularly sensitive substances include p-xylene, toluene, benzene, trichloroethylene, butanol, isopropyl alcohol and the like. Depending on the type of UV lamp, not only a substance having an ionization potential of 10.6 ev or less but also a substance having an ionization potential of 10.6 ev or more can be detected.
Moreover, as the metal electrode 2, there are a SUS (stainless steel) electrode, a gold electrode, a platinum electrode, a copper electrode, and the like, and the kind is not particularly limited. Note that since SUS has good reactivity, the electrode surface is likely to be altered, and therefore, a SUS electrode, a gold electrode, and a platinum electrode are preferable.

The UV lamp 4 may be a generally used lamp in which crimpton, xenon, argon or the like is enclosed.
As an example, when toluene gas is ionized, the UV lamp 4 uses a crimpton-enclosed lamp having an ionization potential of 10.6 ev, and the UV lamp excitation circuit 5 has a frequency of 13.56 MHz that can excite the crimpton. Is an oscillation circuit.
In addition, as described in non-patent literature, there is a method of exciting using an electrode as a method of generating UV.

FIG. 2 shows a block diagram of a photoionization detector according to another embodiment of the present invention.
Further, the photoionization detector of FIG. 2 applies an AC voltage to the detection chamber 1 into which the VOC measurement fluid FL is introduced and discharged, the metal electrode 2 provided in the detection chamber 1, and the resistance to the resistance. An AC applying circuit 3 provided with R, a UV lamp 4 for irradiating the measurement fluid in the detection chamber 1 with short-wave ultraviolet light, an excitation circuit 5 for exciting the UV lamp 4, and a detection signal of a current generated in the resistor R Is synchronized with the wavelength of the AC applying circuit 3 to remove noise, and the phase detector 10 for converting the noise-removed signal into a current value, and the VOC from the current value measured by the phase detector 10 It comprises an arithmetic unit 8 or the like that performs a calculation for conversion into a concentration.
That is, the photoionization detector of FIG. 2 differs from the photoionization detector of FIG. 1 in that a phase detector 10 is used instead of the measurement circuit 7, and the other configurations are the same.

When an AC voltage is applied to the metal electrode 2 by the AC application circuit 3, the metal electrode 2 functions as a capacitor, so that a current depending on the dielectric constant of air flows through the AC application circuit 3.
The VOC in the measurement fluid is ionized by ultraviolet (UV) irradiation, the electric capacity of the metal electrode 2 changes, and the ionic current is generated when ions or electrons are attracted to the metal electrode 2 to change the current value. To do.
Depending on the type and concentration of the VOC to be measured, the output is not stable due to noise or drift.

However, when the current or voltage flowing through the metal electrode 2 is measured by the phase detector 10, that is, when the phase of the AC application circuit and the phase of the detection signal are synchronized by the phase detector 10, the signal that is not synchronized is removed and output. Is stable.
Then, the current value before ion generation is subtracted from the current value at the time of ion generation, and the current value due to ionization is measured. Further, the current value is output as a VOC concentration by multiplying the coefficient for each substance by the calculator 8.
Therefore, as a result, the concentration of VOC can be measured with high accuracy.

FIG. 3 is a block diagram of a photoionization detector according to another embodiment of the present invention.
The photoionization detector of FIG. 3 includes a detection chamber 1 into which the VOC measurement fluid FL is introduced and discharged, an insulating film-covered metal electrode 11 provided in the detection chamber 1 and having a metal electrode 2 covered with an insulating film, An AC voltage is applied to the insulating film covering electrode 11, the AC application circuit 3 provided with the resistance R, the UV lamp 4 that irradiates the measurement fluid in the detection chamber 1 with ultraviolet light having a short wavelength, and the UV lamp 4 are excited. A phase detector for amplifying a current signal generated in the excitation circuit 5 and the resistor R, synchronizing the amplified detection signal and the wavelength of the AC applying circuit 3, and converting the noise-removed signal into a current value 10 and an arithmetic unit 8 that performs calculation for converting the current value into the VOC concentration.

That is, the photoionization detector shown in FIG. 3 differs from the photoionization detector shown in FIG. 2 in that both metal electrodes 2 are insulated and the other configurations are the same. It should be noted that the present embodiment includes not only covering both of the two metal electrodes 2 but also covering only one of them.
Since the metal electrode 2 of the alternating current application electrode can detect a change in the electric capacity of the electrode 2 due to the ionized gas, even if the insulating film-coated electrode 11 covered with the insulating film is used, the electric current changes due to the change in the electric capacity. The value can be measured. Since the insulating film portion coated with the insulating film prevents the electrode from being contaminated by generation of an active substance by UV irradiation or mixing of a corrosive substance, long-term maintenance-free operation is possible. Even if the metal electrode 2 is used, the metal part of the electrode is not deteriorated by being covered with the insulating film, so that the metal electrode 2 that is maintenance-free for a long period of time can be obtained.

Example 1 shows an example using the photoionization detector of FIG.
FIG. 4 shows the measurement results comparing the sensitivity difference between the DC electrode and the AC electrode due to contamination using the photoionization detector of FIG. 1 and the conventional photoionization detector of FIG.
In order to artificially create the condition that the contamination of the insulator covers the electrodes, two metal electrodes 2 (parallel plate electrodes, 30 mm long, 30 mm wide, 0.1 mm thick, distance between electrodes: 0.8 mm, material: One of SUS304) is covered with an insulating film (trade name Parafilm, manufactured by American National Can Company), and the other electrode 2 is covered with a normal metal electrode 2 so that the light shown in FIG. With the ionization detector and the conventional photoionization detector shown in FIG. 9, the UV irradiation time is set to 5 minutes to 10 minutes, and the measurement circuit 7 (ammeter A) is connected in series with the electrode 2 so that each current value (nA ) Was measured.

  As a measurement sample, 100 ppm of toluene gas was used, and UV light having a wavelength of 117 nm was emitted from a UV lamp 4 (10.6 eV, Heraeus, PKR106) excited by a UV lamp excitation circuit 5 (Hereus, C210RF) from 5 minutes. Irradiation was for 15 minutes (irradiation time was 10 minutes). The AC application circuit 3 (function generator, manufactured by Techio Co., Ltd., FG-281, oscillation waveform: sine wave) has a frequency of 280 hertz (Hz), and the AC application circuit 3 and the DC application circuit 13 (Agilent Technology). The applied voltage of E3630A) manufactured by the company was set to 10V. In this example, the current was directly measured by the ammeter A without using the resistor R even in the case of direct current and alternating current.

The inside of the detection chamber (inside dimensions: 200 mm long, 200 mm wide, 125 mm high, material: SUS304) 1 is replaced with the toluene gas (100 ppm), and current is applied to the metal electrode 2 by the AC applying circuit 3 or the DC applying circuit 13. The current generated in the applied electrode 2 was measured as a current value by the measurement circuit 7.
A multimeter (manufactured by Agilent Technologies, U1252A) was installed as the ammeter A of the measurement circuit 7, the current value was measured once every 5 seconds, and the average value for 30 seconds (for 6 times) was plotted.

According to FIG. 4, when the non-irradiation (0 to 5 minutes, 15 to 20 minutes) and the irradiation time (5 to 15 minutes) of the UV lamp 4 are compared, when the DC application circuit 13 is used (indicated by a triangle), There was no difference in current value between when the UV lamp 4 was not irradiated and when it was irradiated, and the current value was almost zero. From this result, it was confirmed that even when contamination occurred only in one of the metal electrodes 2, an accurate current value could not be measured and the measurement sensitivity was lowered.
On the other hand, when the AC application circuit 3 is used (indicated by a black circle), there is a difference in the current value between when the UV lamp 4 is not irradiated and when it is irradiated, so that contamination occurs on one of the metal electrodes 2. However, it was confirmed that the current value can be measured without losing the measurement sensitivity. The generation of current in the AC application electrode occurs not only by the ionic current but also when the metal electrode 2 itself functions as a capacitor. And even if contamination accumulates in one electrode 2 among the two metal electrodes 2 to which an alternating current is applied, the VOC ionized by the other electrode 2 can be captured, thereby preventing a decrease in measurement sensitivity. .

Example 2 shows an example using the photoionization detector of FIG.
FIG. 5 shows the results of measuring the current value to confirm the presence or absence of noise using the photoionization detector of FIG.
As a phase detector 10 of the photoionization detector shown in FIG. 2 in which a resistor R is connected instead of the measuring circuit 7 (ammeter) and the voltage across the resistor R is measured, a lock-in amplifier (NF circuit design) A device similar to that of Example 1 was used except that 5610B) manufactured by Bloc was used.
In addition, as a measurement condition, a normal metal electrode 2 (parallel plate electrode, vertical 30 mm, horizontal 30 mm, thickness 0.1 mm, distance between electrodes: 1.6 mm, material: SUS304) that does not cover anything is used. The setting of the lock-in amplifier 10 and the measurement conditions were an applied voltage frequency of 280 Hz, a time constant of 1 second, and a bandpass filter Q (ratio of center frequency to bandwidth) = 30.

  Similar to Example 1, 40 ppm of toluene gas was used as a measurement sample, and the measurement sample was irradiated with ultraviolet light having a wavelength of 117 nm from a UV lamp 4 excited by a UV lamp excitation circuit 5 for 5 to 15 minutes (irradiation time was 10 minutes). . The applied voltage of the AC applying circuit 3 was set to 10 V, a signal of current generated in the resistor R (10 kilohms) was amplified by the lock-in amplifier 10, and the amplified signal was measured as a current value. Further, the phase difference (degree) between the amplified signal and the wavelength of the AC applying circuit 3 was measured.

When the UV lamp 4 is not irradiated, the current signal flowing through the metal electrode 2 has a phase difference of about 90 degrees with respect to the current signal flowing through the AC applying circuit 3. Therefore, by irradiating the toluene gas with ultraviolet rays by the UV lamp 4, the degree of deviation from 90 degrees, that is, the deviation (Δφ) from 90 degrees was measured.
These current values and phase differences were measured once every 5 seconds, and the average values for 30 seconds (six times) were plotted.

In FIG. 5, the detected current value (nA) is shown on the left vertical axis, and the phase difference (degree) between the detection signal of the metal electrode 2 and the wavelength of the AC applying circuit 3 is shown on the right vertical axis. The current output value (UV off) is indicated by a white circle, the current output value (UV on) is indicated by a black circle, the phase difference (UV off) is indicated by a white triangle, and the phase difference (UV on) is indicated by a black triangle. Indicated.
From FIG. 5, when the UV lamp is not irradiated (from 0 to 5 minutes, from 15 to 20 minutes) and at the time of irradiation (from 5 to 15 minutes), the detected current value plots are aligned substantially in a straight line, causing noise. It was confirmed that there was a difference in current value between non-irradiation and irradiation. This difference in current value is due to the sum of the ionic current and the change in the capacitance of the metal electrode 2. At the time of UV irradiation, the metal electrode 2 functions as a parallel circuit of a capacitor and a resistor. Therefore, the phase of the current is advanced in the range of 0 degree or more and less than 90 degrees from the phase of the AC application circuit 3. In addition, since the metal electrode 2 completely functions as a capacitor when UV is not irradiated, the phase of the current is about 90 degrees ahead of the phase of the AC applying circuit 3.

From FIG. 5, it can be seen that there is a phase difference (deviation from 90 degrees) between the amplified detection signal and the wavelength of the AC applying circuit 3 when the UV lamp is not irradiated and when irradiated. I was able to confirm. It was also confirmed that the plot representing the phase difference was aligned on a straight line between the non-irradiation and the irradiation of the UV lamp, and measurement was possible without causing noise.
As described above, when the photoionization detector of FIG. 2 is used, VOC can be detected not only by the detected current value but also by the above phase difference, so of course when the DC application circuit 13 is used. Compared with the case where the photoionization detector of FIG. 1 is used, the VOC concentration can be measured more accurately.

FIG. 6 shows the relationship between the VOC concentration and the current value difference and the relationship between the VOC concentration and the phase difference when the photoionization detector of FIG. 2 is used.
As measurement samples, toluene gas (4, 40, 100 ppm) and butanol gas (40, 100 ppm) were used, and ultraviolet rays having a wavelength of 117 nm were emitted from the UV lamp 4 excited by the UV lamp excitation circuit 5 to each measurement sample for 5 minutes to 10 minutes. Irradiation was performed (irradiation time was 10 minutes). The applied voltage of the AC application circuit 3 is 10 V, the setting of the lock-in amplifier 10, and the measurement conditions are applied voltage frequency 280 Hz, time constant 1 second, band pass filter Q (ratio of center frequency to bandwidth) = 30, The lock-in amplifier 10 amplifies the signal of the current generated in the resistor R (10 kilohms), measures the amplified signal as a current value, obtains the current value difference between the UV irradiation time and the non-irradiation time. The relationship (calibration curve) between the value difference and the concentration of each substance was expressed. At the same time, the phase difference (degree) between the signal and the wavelength of the AC application circuit 3 was measured by the lock-in amplifier 10 to express the relationship (calibration curve) between the phase difference during UV irradiation and the concentration of each substance. The power value difference is indicated by a solid line, and the phase difference is indicated by a dotted line. Also, the current output value difference of toluene gas is indicated by a black circle, the phase difference (UV on) is indicated by a white circle, the current output value difference of butanol gas is indicated by a black triangle, and the phase difference (UV on) is indicated by a white triangle. Indicated.

From FIG. 6, there is a proportional relationship between the concentration of each substance, the current output value difference, and the phase difference. Thus, if a calibration curve is obtained for each substance, an unknown concentration can be measured.
As an actual measuring instrument, the VOC concentration can be obtained by multiplying the current output value and the phase difference by the coefficient for each substance by the calculator 8.

Further, FIG. 7 shows a measurement result comparing the sensitivity difference between the DC electrode and the AC electrode due to contamination using the photoionization detector of FIG. 2 and the conventional photoionization detector of FIG.
Similarly to Example 1, in order to create a pseudo condition that the contamination of the insulator covers the electrode, one of the two metal electrodes 2 (same as Example 1) is covered with an insulating film (parafilm), Using a normal metal electrode 2 that does not cover anything on the other electrode 2, the photoirradiation detector of FIG. 2 and the conventional photoionization detector of FIG. Each current value (nA) was measured.

  Note that a lock-in amplifier (NF Co., Ltd.) is used as the phase detector 10 of the photoionization detector of FIG. 2 in which a resistor R is connected instead of the measuring circuit 7 (ammeter) and the voltage across the resistor R is measured. An apparatus similar to that of Example 1 was used except that 5610B) manufactured by Circuit Design Block was used. The measurement conditions were an applied voltage frequency of 280 Hz, a time constant of 1 second, and a band-pass filter Q = 30. A current value was measured once every 5 seconds, and an average value for 30 seconds (six times) was plotted.

According to FIG. 7, when the non-irradiation (0 to 5 minutes, 15 to 20 minutes) of the UV lamp 4 is compared with the irradiation time (5 to 15 minutes), when the DC application circuit 13 is used (indicated by a triangle), There is no difference in the current value between when the UV lamp 4 is not irradiated and when it is irradiated, the current value is almost zero, and even when contamination occurs only on one of the metal electrodes 2, an accurate current value should be measured. It was confirmed that the measurement sensitivity was lowered.
On the other hand, when the AC application circuit 3 is used (indicated by a black circle), there is a difference in the current value between when the UV lamp 4 is not irradiated and when it is irradiated, so that contamination occurs on one of the metal electrodes 2. However, it was confirmed that the current value can be measured without losing measurement sensitivity. The generation of current in the AC application electrode occurs not only by the ionic current but also when the metal electrode 2 itself functions as a capacitor. And even if contamination accumulates in one electrode 2 among the two metal electrodes 2 to which an alternating current is applied, the VOC ionized by the other electrode 2 can be captured, thereby preventing a decrease in measurement sensitivity. .

Compared to the case shown in FIG. 4 using the photoionization detector shown in FIG. 1 (Example 1), there is less variation in plots during irradiation of the UV lamp 4, and the phase detector 10 generates noise. It was confirmed that it was removed.
Depending on the type and concentration of the VOC to be measured, a minute signal due to ionization may be buried in noise, but noise can be removed by measuring the current or voltage flowing in the metal electrode 2 with the phase detector 10. Therefore, it is possible to measure the VOC concentration with high accuracy.

Example 3 shows an example in which the photoionization detector of FIG. 3 is used.
FIG. 8 shows the result of measuring the current value using the photoionization detector of FIG. 3 in order to confirm the presence or absence of noise as in Example 2 (FIG. 5).
As measurement conditions, a metal electrode 2 (parallel plate electrode, 30 mm long, 30 mm wide, 0.1 mm thick, distance between electrodes: 0.8 mm, material: SUS304) and an insulating film-covered electrode 11 (both two metal electrodes 2) Was created with the condition that the metal electrode 2 was covered with an insulating film in a pseudo manner, and the toluene gas concentration was changed to 100 ppm, the frequency of the applied voltage was changed to 1 kHz, and the resistance R Was set to the same conditions as in Example 2 (FIG. 5) except that the irradiation time was 5 minutes.

As a measurement sample, 100 ppm of toluene gas was used, and the measurement sample was irradiated with ultraviolet rays having a wavelength of 117 nm from the UV lamp 4 for 5 to 10 minutes (irradiation time was 5 minutes). Further, the applied voltage of the AC application circuit 3 is set to 10 V, the signal of the current generated in the resistor R (5 ohms) is amplified by the lock-in amplifier 10, the amplified signal is measured as a current value, and further amplified. The phase difference (degree) between the signal and the wavelength of the AC application circuit 3 was measured.
These current values and phase differences were measured once every 5 seconds, and the average values for 30 seconds (six times) were plotted.

Similarly to FIG. 5 of the second embodiment, the detected current value is shown on the left vertical axis, and the phase difference between the detection signal amplified by the lock-in amplifier 10 and the wavelength of the AC applying circuit 3 is shown on the right vertical axis. .
As shown in FIG. 8, when the UV lamp is not irradiated (from 0 to 5 minutes, from 10 minutes to 15 minutes) and at the time of irradiation (from 5 minutes to 10 minutes), the plots are almost side by side, and output is performed with and without UV lamp irradiation. It was confirmed that a difference occurred, and it was confirmed that the VOC concentration could be measured even with the electrode using the insulating film-coated electrode 11.

In addition, it is considered that when both the electrodes 2 are covered with the insulating film, no ionic current is generated, and the output is changed only by the change in the electric capacity of the electrode 2.
Further, in the case of the AC application circuit 3, since the metal electrode 2 functions as a capacitor, the electric capacity of the metal electrode 2 changes and a phase difference occurs between the detection signal and the wavelength of the power supply of the AC application circuit 3, and the phases of both of them change. Shift.

Also from FIG. 8, as in FIG. 5, there is a phase difference between the detection signal amplified by the lock-in amplifier 10 and the wavelength of the AC applying circuit 3 when the UV lamp is not irradiated and when irradiated. As a result, gas ionization was confirmed.
As described above, since the metal electrode 2 applied with alternating current can detect a change in the capacitance of the metal electrode 2 due to the ionized gas, the current value is measured by the change in the capacitance even when the insulating film is covered. it can. The insulating film covered with the insulating film prevents the electrodes from being contaminated by the generation of active substances or corrosive substances caused by UV irradiation, so there is no accumulation of contamination in the electrode conductors and long-term maintenance-free operation. Is possible. Moreover, even if the metal electrode 2 is used, the metal part of the electrode does not deteriorate.

  The present invention can be used not only in the environmental field but also in the industrial field as a VOC concentration measuring apparatus and measuring method.

DESCRIPTION OF SYMBOLS 1 Detection chamber 2 Metal electrode 3 AC application circuit 4 UV lamp 5 UV lamp excitation circuit 7 Measurement circuit 8 Calculator 10 Phase detector (lock-in amplifier)
11 Insulating film coated electrode 13 DC application circuit

Claims (6)

  A detection electrode for detecting a volatile organic compound in the measurement fluid, an application means for applying an alternating voltage or an alternating current to the detection electrode, and an ultraviolet ray on the measurement fluid for ionizing the volatile organic compound in the measurement fluid A photoionization detector for a volatile organic compound, comprising: a UV lamp for irradiation; an excitation circuit for exciting the UV lamp; and a measuring means for measuring a current or voltage flowing through the detection electrode.   2. The photoionization detector according to claim 1, wherein the measuring means is a phase detector.   The photoionization detector according to claim 1 or 2, wherein the detection electrode is covered with an insulating coating.   An AC voltage or an AC current is applied to a detection electrode for detecting a volatile organic compound in the measurement fluid, and the measurement fluid is irradiated with ultraviolet rays to ionize the volatile organic compound in the measurement fluid, and the detection electrode A method for photoionization detection of a volatile organic compound, characterized by measuring a current or a voltage flowing through the substrate.   5. The photoionization detection method according to claim 4, wherein a current or voltage flowing through the detection electrode is measured by phase detection.   6. The photoionization detection method according to claim 4, wherein the detection electrode is an insulating coating.

Images